Compounds polyhedral boranes

The area of chemistry involving the polyhedral boranes and carboranes has seen enormous growth in recent years. Accordingly, the brief survey here does not include many interesting facets of these fascinating compounds. For more details on the polyhedral boranes, the references at the end of this chapter should be consulted. 

Heteroatom Cluster Compounds Incorporating Polyhedral Boranes as Ligands... 

This, together with the Ir-H terminal bond indicates that the compound can be regarded as a further example of the growing number of compounds in which the high oxidation state Ir(v) is stabilized by coordination to a soft polyhedral borane ligand... 

This mechanism bears a close relationship to the polymerization of polyhedral boranes. Certainly there is clear evidence for the formation of Fe(CO)4 from Fe(CO)5. The production of Fe2(CO)9 from Fe(CO)5 is catalyzed by light, but further polymerization of Fe2(CO)9 to Fe3CO)I2 is not observed under these conditions. For the related osmium systems, there is some evidence for the formation of 082(00)9 from Os(CO)5, but irradiation of Os3(CO)J2 does not produce higher clusters although in the presence of molecules such as CO or CjH L) the compounds... 

Another very useful rule for classifying the structures of polyboranes and hetero-boranes as well as many metal boron cluster compounds and their derivatives has been developed by Rudolph, Williams, Mingos and Wade (see Chapter 1.1.2) [4]. Today these are generally termed the Wade rules. They can be derived from the structures and electronic requirements of closed polyhedral boranes, such as an octahedron or an icosahedron, which are present in the anions B6H62 and B,2 H, 22. Since there are only exopolyhedral B-H bonds the number of electron... 

In contrast to the polyhedral boranes B H +m there exist a number of neutral boron duster molecules B X (X = Cl, Br, I, NR2, R) all of them having closed deltahedral structures in spite of the fact that the number of bonding electron pairs is only n. For this reason these homonudear cluster compounds of boron are called hypercloso polyboranes. However, there also exist anions of type B X 2 which fit Wade s rules. 

The boron hydrides, including the polyhedral boranes, heteroboranes, and their metaUa derivatives, encompass an amazingly diverse area of chemistry. This class contains the most extensive array of structurally characterized cluster compounds known. Included here are many novel clusters possessing idealized molecular geometries ranging over every point group symmetry from identity (C[) to icosahedral (I[). Because boron hydride clusters may be considered in some respects to be progenitorial models of metal clusters, their development has provided a framework for the development of cluster chemistry in... 

Another class of molecules that will be discussed contains cluster compounds such as the polyhedral borane anions, B H -", and some metal containing species such as the metal carbonyl clusters. 

Table 13 4 4. Application of the mno rule for electron counting in some condensed polyhedral boranes and related compounds ...

A special case of molecular boron suboxides is found in the hydroxylated polyhedral boranes (see Boron Hydrides). For example, the icosahedral anion closo- Q 2H 2 and related compounds can be completely hydroxylated to form discrete molecular boron suboxides, including closo-[Bi2(OH)i2] These can be regarded as members of a class of formally hydrated suboxides related to compounds snch as tetrahydroxydiborane, B2(HO)4 (BO H2O). The terminal hydroxyl groups of these compounds can be fimctionahzed as esters or ethers to produce a variety of large molecule derivatives. 

In discrete molecules, the elements of group 13 are most typically found in the +3 oxidation state. There are major exceptions to this rale, such as the polyhedral boranes (see Boron Hydrides) and carboranes (see Boron Polyhedral Carboranes and Boron Metallacarbaboranes), which formally contain and are outlined elsewhere in this Encyclopedia, and the compounds of thallium (Tl), which typically contain Tl for the reasons described above. Thus, in this section, compounds containing group 13 elements in the atypical -I-1 and -1-2 oxidation states are presented. 

Polyhedral hydroborate anions (commonly referred to as polyhedral borane anions in the current literature) of generic formula have assumed an unusual status in inorganic chemistry within recent years, not only because of their high thermal and hydrolytic stability, but also because of the wide range of geometries which the boron atom skeletons in compounds of the series (n = 6-12) can adoptd Convenient syntheses... 

Although it may not be surprising that the same set of electron-counting rules can be used to describe satisfactorily such similar compounds as boranes and carboranes, we should examine how far the comparison can be extended. Can Wade s rules, for example, be used effectively on compounds containing metals bonded to boranes or carboranes Can the rules be extended even further to describe the bonding in polyhedral metal clusters ... 

A theoretical study of the intermediates involved in the formation of phospha-propyne from pyrolysis of vinylphosphirane has led to a new route to phospha-alkynes. Thus, pyrolysis of trimethylsilyl(l-phosphiranyl)diazomethane has yielded MeaSiC = P, via an intermediate 1-phosphiranylmethylene . Regioselec-tivity in the [3 + 2] cycloaddition reaction between phosphaethyne and diazomethane has been studied by theoretical techniques , and further examples of reactions of this type described . Cycloaddition of phospha-alkynes with silylenes has also been reported. The primary phosphine 324 has been isolated from the addition of diethylphosphite to t-butylphosphaethyne. The chemistry of phospha-alkyne cyclotetramer systems has been reviewed and the first examples of platinum(II) complexes of such cage systems described. Aspects of the reactivity of coordinated phospha-alkynes have received further study, and a remarkable metal-mediated double reduction of t-butylphosphaethyne to the complexed fluorophosphine 325 described Phosphorus-carbon-aluminium cage structures have been isolated from the reactions of kinetically stable phospha-alkynes with trialkylaluminium compounds and new phosphaborane systems have been obtained from the reactions of phospha-alkynes with polyhedral boranes . Further studies of wo-phospha-alkyne coordination chemistry have appeared . The reactivity of the ion 326 has been explored. ... 

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